Estimates of Climate Change Effects on Monthly Stream Runoff Applications to Streams in Minnesota and Oklahoma

Abstract

This study examines the relationship between runoff and climate in two small watersheds in the mid~continental U.S. A parametric runoff model is applied to two watersheds with substantially different climates. One watershed is in the north~central U.S. with cold climate, heavily timbered, with an annual precipitation of 768 nun, an annual runoff of 448 mm and mean annual temperature of 3.5 DC. The other is in the south-central U.S. with warm climate, mainly covered with pasture and agricultural crops, with ,an annual precipitation of 746 nun, an annual runoff of 49 mm and mean annual temperature of 15°C. The parametric runoff model has a monthly time scale and simulates the historical monthly runoff of these watersheds with an R2 of 0.95 (Mohseni and Stefan, 1996). In order to investigate the effects of potential climate change on runoff from the two watersheds the parametric runoff model was applied with past and projected 2xCO2 climate scenarios as input. The output of two General Circulation Models (GCMs) was used to specify the 2xC02 climate scenarios. One GCM comes from the Goddard Institute of Space Studies (GISS) and the other from the Canadian Climate Center (CCC). Unfortunately the two GCMs project different precipitation under the 2xC02 scenarios for the northern watershed and therefore the projected stream runoff is also not fully consistent. In the northern watershed more runoff is projected to occur in winter under a warmer climate and less runoff in spring. A 24% increase in annual precipitation, mainly occurring in fall, causes about 12% increase in annual runoff under the GISS 2xC02 climate scenario. Under the CCC 2xC02 climate scenario, virtually no increase in annual precipitation is predicted, and hence a 5% decrease in annual runoff is projected; also, the runoff distribution throughout the year is projected to change. For the southern watershed, the GISS and CCC 2xC02 climate scenarios are in agreement. A 6% increase in annual precipitation, mainly projected to occur in fall, results in a 6% increase in annual runoff according to the parametric runoff model. Runoff in the southern watershed is currently only a small fraction of precipitation (less than 7%), Depending on the season in which precipitation or surface air temperature increases, the change in runoff differs. An increase in spring precipitation can cause a significant increase in the direct runoff, whereas an increase in fall precipitation causes an increase in the base flow later in fall and winter. This study has shown that the runoff-precipitation relationship in warm and seasonally dry regions is very different from that in temperate to humid climate regions. Therefore, runoff responses to climate changes are also substantially different.